By Harris | Technical Team, FUSHUN METAL
From Scrap to Spec: The Purification Chain That Makes Superalloy Revert Virgin-Grade
A common assumption in procurement circles is that recycled metal is inherently inferior metal. Our position at FUSHUN METAL is the opposite: revert is not worse metal — it is contaminated metal. Contamination is removable; the alloy chemistry itself is already correct, because it was melted and controlled once before. With disciplined purification, remelted superalloy meets virgin specification — and with each remelt it can actually become cleaner, the effect the industry calls “purer with every melt”.
A note on designations before we begin: two Chinese grades appear in this guide. GH4169 corresponds to Inconel 718 in the United States (UNS N07718) and to European material 2.4668 (NiCr19Fe19Nb5Mo3); GH4738 corresponds to Waspaloy (UNS N07001) and to European material 2.4654 (NiCr20Co13Mo4Ti3Al). Wherever a Chinese designation appears below, its US and European counterparts are given alongside, and a full cross-reference is summarized in Table 4.
Why the gates and risers are your dirtiest scrap
During melting and pouring, high-temperature oxidation and refractory contamination form oxide films such as Al2O3, non-metallic inclusions such as SiO2, and surface pollutants including oil and ceramic core residue. Because gating and riser regions are the last to solidify, these contaminants concentrate exactly where the return material comes from. Remelting such revert directly cuts high-temperature stress-rupture life by 25–40% — a catastrophic, silent hit to part quality. Published studies on GH4738 (Waspaloy) confirm the mechanism: revert addition raises impurity content and inclusion counts, sharply degrades tensile ductility, collapses stress-rupture life, and drives MC carbides to aggregate at grain boundaries; sulfur, in particular, segregates to grain boundaries and weakens them. The conclusion is not negotiable: purification before melting is a gate, not a preference.
Sort before you clean: three-tier grading
The first discipline is classification. Revert must be segregated by alloy grade first — mixing chips of different compositions can fail the chemistry of an entire ingot. Within one grade, material is then graded by oxidation and inclusion state, and each tier receives a proportional cleaning intensity, as shown in Table 1. Machining chips additionally pass through magnetic screening to separate ferrous pick-up. Cleaning effect is verified by the mass-loss method and by XRF surface analysis of characteristic contaminant elements.
| Tier | Condition of the material | Required cleaning |
|---|---|---|
| Level 1 | Severe shrinkage porosity in the gating system (continuous voids or looseness) and heavy inclusions (filter mesh or residual shell) | Repeated compound treatment: 4–6 cycles of vibration, shot blasting and grit blasting |
| Level 2 | Shrinkage porosity and inclusions present in the gating system | 2–3 cycles of vibration, shot blasting or grit blasting |
| Level 3 | No obvious shrinkage or inclusions; clean scrap parts, uncontaminated obsolete material, pure machining chips | Shot blasting or mechanical grinding until bare metal with no visible residue |
The chemistry of cleaning: composite baths that do not eat the base metal
Physical removal cannot touch the dense oxide films and the inorganic, organic and oily contaminants penetrated below the surface. These must be converted into soluble salts by chemical reaction. The key is selecting the right bath and duration: aggressive enough to strip films, gentle enough not to corrode the alloy matrix. The answer is composite systems — an alkaline bath compounded from sodium hydroxide and sodium carbonate, and an acid bath compounded from nitric acid and aminosulfonic acid. Composite baths win on two fronts: different components act through different dissolution mechanisms for a synergistic effect, and lower individual concentrations cut the risk of harmful-element pick-up from the bath itself. Table 2 summarizes the operating envelope used in practice.
| Parameter | Setting |
|---|---|
| Alkaline bath | Sodium hydroxide + sodium carbonate composite |
| Acid bath | Nitric acid + aminosulfonic acid composite |
| Washing sequence | Alkali wash, water rinse, acid wash, water rinse |
| Bath temperature | 20–35 °C, maintained by hot-water bath |
| Ultrasonic assist | 15–40 kHz oscillation |
| Final rinse | Pure water at 30–40 °C for 25–35 min |
| Drying | 150–200 °C oven for 30–45 min |
Residual contamination is quantified by AAS, ion chromatography or titration, judged against GJB 8781.16-2015 and HB 5220. Orthogonal experiments are used to lock the concentration–temperature–time window that maximizes cleaning efficiency.
Vacuum purification: chasing the last gas molecules
After chemical cleaning, the surface still adsorbs H2, O2 and CO2 plus residual cleaning fluid. These leave only under high temperature and high vacuum, through the adsorption–diffusion–outgassing cycle. Every control variable is a compromise, as Table 3 shows.
| Variable | Risk if set too high | Risk if set too low |
|---|---|---|
| Heating temperature | Loss of low-melting volatile elements; even matrix phase transformation | Adsorbed gas molecules cannot keep diffusing |
| Holding time | Accelerated diffusion drives composition segregation | Gas does not reach the surface; outgassing incomplete |
| Vacuum level | Maintained as high as practical | Partial pressure stays too high; adsorption–diffusion–outgassing equilibrium never breaks |
Choosing the remelting route: single, duplex or triplex
Before melting, cleaned revert must be sized — typically 5–10 mm — and packed into the crucible with a “dense at the bottom, loose at the top” rule, layer by layer, tamped, with clearance to the wall so expansion does not crack the crucible and the melt column can settle freely. Then the vacuum route is chosen. For international buyers, the Chinese designations cited below map to familiar Western grades (Table 4). Three families dominate practice, as compared in Table 5.
| China (GB) | United States | Europe (EN / DIN) |
|---|---|---|
| GH4169 | Inconel 718, UNS N07718 | 2.4668, NiCr19Fe19Nb5Mo3 |
| GH4738 | Waspaloy, UNS N07001 | 2.4654, NiCr20Co13Mo4Ti3Al |
| Route | Sequence | Key benefit | Main trade-off | Typical fit |
|---|---|---|---|---|
| Single | VIM + ESR | Precise chemistry control; removes part of gases and inclusions | Limited cleanliness and uniformity ceiling | Not ideal for impurity-sensitive, strict-uniformity alloys such as GH4169 (Inconel 718) and GH4738 (Waspaloy) |
| Duplex | VIM + VAR | Deep removal of N, O, P, S and inclusions; clean ingot surface | Al and Ti burn-off of 5–8%; segregation tendency | Cleanliness-critical alloys where Al/Ti burn-off control demand is low |
| Triplex | VIM + ESR + VAR | Combines all advantages; maximum purity and uniformity | Highest cost and longest cycle | Highest cleanliness and property standards |
Triplex melting is not marketing theater — it is measurable. Comparative melting data on large ingots shows that after VIM + ESR + VAR the sulfur content drops from 0.0021% to 0.0007%, oxygen from 0.0017% to 0.0008%, inclusion count from 5,736 to 3,412 per unit area, and inclusion size from 3.1 to 2.5 micrometres. A two-pass VAR practice is common for demanding grades: the first pass runs low current and slow melting so directional solidification pushes impurities to the ingot top for removal; the second pass runs high current and fast melting in a skull furnace to dissolve stubborn high-melting inclusions, with electromagnetic stirring floating inclusions to the surface and slow furnace cooling suppressing grain coarsening. Current, voltage, vacuum, stirring intensity and solidification rate must be controlled together to limit burn-off and segregation.
Proving it: quality gates before release
No regenerated material ships without evidence. Chemical composition is checked by ICP-OES, SEM and EDS at high sampling frequency, with the gas impurities capped as in Table 5. Microstructure is assessed per GB/T 14999, mechanical properties per GB/T 14992, and internal and surface soundness by ultrasonic, X-ray and magnetic-particle NDT per HB/Z 33 and HB 5358.2/5358.3. If composition fails, the response is disciplined: re-test under standard procedure, trace batch and source, adjust melting parameters, and compute precise alloy-element additions.
| Element | Maximum allowable content |
|---|---|
| N | 15 × 10-6 (15 ppm) |
| O | 10 × 10-6 (10 ppm) |
| H | 1 × 10-6 (1 ppm) |
“Purer with every melt” is real — but it is earned by process discipline, not by the word “revert” on a certificate. When a mill offers you recycled-content superalloy, audit the chain: how the scrap was graded, how it was cleaned, which vacuum route was used, and what impurity limits were verified. The entire difference between recycled and virgin-grade recycled lives in that chain.
